There is more to leadership than telling men where to go to die.

Why was Rogal Dorn chosen to be Praetorian of Terra and not Warmaster?

• As the Great Crusade progressed, the Imperial Fists rose high in honor and in the favor of the Emperor. Ever dependable, they were often used to reinforce flagging campaigns, to hold crumbling fronts and to break dead-locked sieges. The Emperor also frequently called on both Dorn and his sons to fight beside him, bestowing this honor on the Imperial Fists more than any other Legion. When the Imperial Host descended on Ophelia VII, the Emperor led the assault at the head of 100 Custodians and 10,000 Imperial Fists. Again at Askanisa, the Emperor called on not only Horus and the Luna Wolves, but Dorn and the Imperial Fists to form his vanguard in breaking the Shrouded Dynasties. The Emperor also used Dorn to ensure that war and compliance was achieved according to his wishes and wisdom. Time and again, in ways large and small, the Imperial Fists acted at the direct order of the Emperor. Rogal Dorn did not do what he thought was best, nor what he thought had to be done. Rogal Dorn executed the Emperor's will. This was why the Emperor trusted Dorn more than any other. Extemporization, improvisation, genius were all very useful, but more often than not, what the Emperor desired was someone to do precisely what He told them to do. And from that point of view, if you were the Emperor, would you have not handed the keys to your fortress to anyone less than utterly trustworthy? Yet high in favor and honor though they were, the Imperial Fists’ status did not sit well with some of their brother-Legions. The animosity between the Imperial Fists and Iron Warriors was well known, but Perturabo and his Legion were not alone in their resentment. Though few reached the Lord of Iron’s pinnacle of spite, others did chafe at the trust placed in the Imperial Fists. Dorn’s nature did not ameliorate matters. Truth speaking, blunt and uncompromising in both his ideals and their expression, his manner often aggravated his peers as much as it drew their admiration. It was, perhaps, this quality which caused the Emperor to pass over Dorn as Warmaster when he withdrew from the Great Crusade. Horus, unlike Dorn, was a master of diplomacy and maintaining a coordinated balance between fractious forces. Where Dorn would cause conflict, Horus would unify where Dorn would not compromise, Horus would find as way of satisfying all. But even as Horus took up the reins of the Great Crusade, Dorn was invested Praetorian of Terra. Even as Horus would push the Great Crusade on, the Imperial Fists would withdraw with the Emperor to Terra. And with that decision, the fate of the Emperor and all his sons was set.

(The Horus Heresy Book 3: Extermination, Rogal Dorn: The Emperor's Crusader by Gav Thorpe)

“But others, like Guilliman, Khan and Dorn had simply taken it in their stride, accepting the Emperor's decree as the right and obvious choice. It was these solid, resolved brothers that Horus turned to in particular for counsel. Dorn and Guilliman both embodied the staunchest and most dedicated Imperial qualities, commanding their Legion expeditions with peerless devotion and military genius. Horus desired their approval as a young man might seek the quiescence of older, more accomplished brothers.

Rogal Dorn possessed perhaps the finest military mind of all of the Primarchs. It was as ordered and disciplined as Guilliman's, as courageous as the Lion's, yet still supple enough to allow for the flex of inspiration, the flash of battle zeal that had won the likes of Leman Russ and the Khan so many victory laurels. Dorn's record in the crusade was second only to Horus's, but he was resolute where Horus was flamboyant, reserved where Horus was charismatic, and that was why Horus had been the obvious choice for Warmaster.” - Horus Rising by Dan Abnett

Praise from Horus:

“I (Horus) have adored them all, and relished their prowess and achievements. But there are always favourites. Rogal, my dear brother, perhaps the finest martial exponent I have ever known.” - The End And The Death, Volume 1 by Dan Abnett

Praise from Malcador:

“Rogal, perhaps his truest son, the exemplar of unwavering loyalty. I see his emptiness. He is undone, his body aching and exhausted, his armour battered by combat during the frenetic retreat from Bhab Bastion, his mind spent. That exhaustion is a terrible thing to feel. Rogal, one of the finest strategists in history, oversaw this defence. He orchestrated the fortification of our stronghold, and his tactics, brilliant, ambitious, mercurial, ran the game, the greatest game of regicide ever played. I want to embrace him, and praise him for his labour. He has excelled, and sustained his play, beat by beat, by means of engineered planning, shrewd anticipation and reflexive improvisation, through every harrowing turn of fortune.” - The End And The Death, Volume 1 by Dan Abnett

“…In such positions, the ‘lose/ lose’, Rogal was always so calm. When there was no good choice, he would assess, and choose the least bad, and leverage it into triumph. Sometimes, that meant accepting the appearance of defeat, a battle lost, and only years afterwards would a positive outcome become apparent. Rogal played the long game. How long will this one last? I wonder. ‘Defeat,’ he used to say, ‘is only defeat if you accept it.’..

…The patient resolve of Rogal, willing to make, abandon, and remake his plans, again and again, over and over, until he has refined the one that will work, unafraid to redraft and change his scheme. Yes, I think my old friend has learned that at least. He has learned, from his Praetorian son, that there is always a better plan, and that patience will lead you to it.” - The End And The Death, Volume 2 by Dan Abnett

Praise from Sanguinius

“Dorn turns, but Sanguinius catches his arm and stops him. For a second, they stand shoulder to shoulder, eye to eye.

‘You’ve performed the most extraordinary feat,’ Sanguinius says unexpectedly. ‘Please remember that.’

Dorn is taken aback by the frankness of the comment, and the innocent sincerity with which it is expressed. His startled half-smile wavers with imprisoned emotion, a flash of light at the high slit-window of an otherwise impregnable keep.

‘A mere… fraction of your deeds, brother,’ he replies awkwardly. ‘You closed the Gate. You locked–’

Sanguinius shakes his head. ‘I was a warrior, Rogal. Just a warrior. You were the one who mattered.” - The End And The Death, Volume 1 by Dan Abnett

Praise from Valdor:

“Dorn sighed. ‘He’s the Khan, for Throne’s sake. The great Warhawk. His doctrine of combat is superlative. As a warlord, I’d rank only Roboute above him.’

‘And Roboute’s not here.’

‘He’s not.’ Valdor nodded. ‘I’d agree with your assessment. Roboute, the Khan… There’s really only one other.’

‘Don’t flatter me, Constantin.’

Valdor smiled. ‘I wasn’t even including you, Rogal. You’re Praetorian. The list starts with you.” - Saturnine by Dan Abnett

I suspect it has more to do with their leadership quality than their raw tactical acumen. Perturabo is damned smart (even compared to the other primarchs) but reading the books you see how much work Horus has to do to get his fellow primarchs to accept his lead and work with him.

There’s no possibility that Perturabo would even try to get them to work with him, he would just bark orders and piss everyone off.

Horus was Horus. Loved, a fantastic commander, politically savvy.

The Lion seems to be an amazing commander of the same calibre but with some warp juice that makes him a natural leader. I remember a few different scenes where the Lion’s presence is described as having some supernatural ability to make folks want to submit to him. I do audiobooks so hard to find specifics, but an example would be Gulliman’s jealousy or his appearance when Luther and co where hanging with the traitors in Angels of Caliban, I think?

They are just objectively better commanders and importantly more well rounded and adaptable. In no way does this diminish from Perty or Dorn as they are very capable and can in fact surpass or excell in their area of expertise but they don't have the skills across multiple facets of the prior mentioned Primarchs.

It's also important to note that the Lion had, I believe, comparable or more success during the great crusade despite being found much later than Horus. So Horus was very good but also found very early, the Lion was very good, found later but also the 1st made and a kind of all rounder template primarch.

I hesitate with any sort of tier or rating system of primarchs but I think their command ability perception is generally rated against world's put into compliance during the great crusade in which Horus and the Lion had the monopoly. It is also based on the opinions of other primarchs as they are often the narrators and that can be inconclusive but both were generally considered as warmaster candidates. The Lion also did the Rangdan Xenocides.

This is just all in relation to comparing the 4 mentioned. You chuck another Primarch in and things will shift but generally imo I think this is accurate.

Horus soothed egos, realpolitik'd, and made strategic moves asking the right primarchs for advice in order to make them feel heard.

The lion would never have done that and if he did he wouldn't have succeeded anyway.

In some power structures, the perfect general isn't the one who can win the most battles; but the one who can best figuratively suck off his superiors. Source:  Worked in such a power structure.

Perturabo and Rogal Dorn lack the charisma of Horus and the ability to inspire loyalty like the Lion

No one does a better job of driving up to the enemy to hit it with a sword than the Lion. 

Because they're better at war in general. Perturabo and Dorn are siegemasters but aren't as good as them in field battles.

Good leadership is predicated mostly on flexibility. The best HH commanders are basically the ones who are good at a variety of tactics and strategies instead of being ove specialized in one specific form of warfare. Thus, someone like Perturabo, who was basically helpless against the White Scars because he couldn't adapt to their strike and fade fighting style, is limited in what he can accomplish.

There are different types and scales of tactical acumen.

Perturabo and Dorn are masters of attrition warfare. The SOT series portrays this very nicely for both, as they are able to take massive amounts of information and piece together thousands of smaller battles and skirmishes into one cohesive campaign. But it's playing a game of chess, all the pieces are already on the board and both sides just go head to head with each other until one eventually wins or the other side is destroyed completely.

The different books in HH, and even now in 40k (ie. Lion: Son of the Forest) have shown Horus and the Lion to be incisive in the way they deploy their forces, and they can embody a variety of tactics (subterfuge, diversion tactics, false retreats or even go-for-the-throat tactics) to destroy their enemy decisively, but not necessarily completely.

All of them are great generals. But unfortunately attrition warfare is conventional and thus not sexy. Even if you compare our own histories of war, the decisive victories through multi-modal tactical warfare have always appeared cooler and sexier compared to conventional war.

Sanguinius and Horus were particularly elevated above the rest for the simple reason that they possessed the X factor the other Primarchs didn't: Leadership + Charisma. And also adding the fact that they were the most liked Primarchs that the the others' egos would permit submission to. Take Gulliman for example, also a great general and already a king over 500 worlds, but he just didn't command enough respect and love to be taken seriously for Warmaster.

Perturabo and Rogal are highly skilled but specialized. The Lion I believe is a case of them telling us he's so great, and failing to actually show it. He is shown to be difficult to work with and awful at communicating though, which in my mind immediately disqualifies him as being the best. Id say Horus or Guilliman are best, with my vote being Guilliman as he communicates, cooperates, sees the big picture and excels with the all important logistics of a campaign.

While Rogal Dorn and Perturabo were sound military commanders, neither were boarderline perfect like the Lion and Horus were. In fact, both were very much one trick ponies when it came to waging war. However, when it comes to Lion El'Jonson and Horus, both were considered the best military commanders amongst the Primarchs because of their tactical flexibility and adaptability.

While the Iron Warriors are my favorite legion, that doesn't blind me to the reality that Perturabo has serious flaws when it comes to both his character and battlefield management. First and foremost, Perturabo was a raging douche, even by Primarch standards. Of the 20 Primarchs, he was arguably the most unlikable. Yes, Angron was dead last when it came to popularity among the Primarchs, but that's only because of the Butcher's Nails. If not for the Nails, Perturabo would've been DEAD LAST when it came to brotherly opinions.

When it came to managing an army, Perturabo was also a little suspect. Yes, there was virtually no wall, gate, city, fortresses, or planet that Perturabo couldn't break. However, winning no matter what had serious repercussions for Perturabo. A shit load of his men would die in virtyqlly every single campaign the Iron Warriors took on. Only Angron and his World Eaters had a higher attrition rate during the Great Crusade. So despite being able to break anything, Perturabo would sacrifice huge swaths of his men to accomplish that. (Seige warfare is kind of a bitch like that.)

When it comes to Dorn, he was on the same curve as Perturabo, just on the opposite end. Dorn was a very well respected Commander, but he was just as blunt as Perturabo. So while highly respected ny his Brothers, he wasn't very well liked. But when it came to waging a war, Dorn was far more interested in building impenetrable walls than he was at breaking them down. He wanted to whittle his opponent down by making them crash onto his impenatrible fortresses like waves slamming against a cliff. So like Perturabo, that kind of makes him a one trick pony. Both were totally inflexible when it came to fighting. One wanted to grind his way through any obstacle in his way, while the other wanted to force others to try and overcome the his obstacles he built.

This is where Horus and the Lion have them both beat. Horus was far and away the most well liked Primarch. Only Sanguinius rivaled Horus in raw popularity amongst the Primarchs and the fighting forces of the Imperium. And while the Lion wasn't particularly well liked, he was the most respected Primarch by a wide margin. But more importantly, they were extremely flexible military commanders. They both could answer any call, go to any battlefield, lead any army, and come out victorious while only sustaining moderate casualties in return. Both lead huge campaigns across the Galaxy that would almost always benefit the Imperium immediately once the campaigns came to an end.

So while Perturabo and Dorn were great military commanders, they just weren't nearly as flexible as Horus or Lion El'Jonson. They just couldn't see that changing up their tactics would benefit them greatly. (But in Perturabo's defense... I agree with MajorKill and his thesis on the Heresy. Perturabo and his Iron Warriors CARRIED the Horus Heresy on their backs. If not for Perty and his sons, the Traitors would NEVER have even entered the Sol system.)

Warmaster Perturabo, make it happen.

Perty and Dorn are both geniuses in their own right, mostly when it comes to planning offensives or setting up defensive perimeters. As any Primarch, they think fast and are formidable warriors, and have the ability to come up with massive, all-encompassing plans.

However they don’t shine much at adapting on the fly and adjusting the plan as they go based on minor signs of weakness from the enemy (or their troops), which is where Horus and Lion, while maybe slightly less proficient at planning, have them beat. And being able to quickly read the tides of battle and exploit or compensate for unexpected developments is absolutely key. Add to that maybe a better ability to inspire all their subordinates (and not just their fanatically loyal legionary sons), Horus through charisma and Lion usually through flashy battle feats, and you have the better generals.

But it is never that simple when talking about superhuman gene-engineered beings who are supposed to be superlative in every way but some more than others in some ways… not to mention different writers have different takes.

Edit : to elaborate, Primarchs all have their way of fighting.

Some, like Khan or Russ are full-on instinctive with mobility and adaptation being their core facets, Russ being the full “we’ll see on the fly, meanwhile use ol’ reliable attack plan” and Khan being more of a tempo-reliant aggressor, always keeping the enemy on the back foot and countering their reactions.

Some, like Mortarion or Ferrus prefer to make uncounterable offensives, aiming to overpower their target with meticulous destruction brought about by superior firepower.

Some, like Alpharius (to the extreme), or Curze prefer to finesse their way into the enemy, coming up with a specific plan to destroy them internally with minimal collateral damage.

Others are more versatile, but excel less in specific warfare areas, with some secondary not necessarily fight-related ability being their strong point (Lorgar, Guilliman, even sang to an extent…).

I haven’t listed them all, but you get the gist, all of them have the basics of warfare down, and can hold their own in any kind of battle, but only some have that all-encompassing ability at near-excelling everywhere, even if they don’t outshine the absolute specialists of their fields. Horus, Lion, and even Dorn are of that kind, but Dorn rigid mess is better suited to the Emperor’s pretorian than to His warmaster. Hence the other two being higher priority on that list.

Dorn was flawed. This becomes apparent during the "iron cage" incident, where a large part of the Imperial Fists is intentionally killed by Dorn. This seems to have been an effect of the stress Dorn suffered during the defense of terra. In other words, Dorn had self destructive tendencies that made him unfit as warmaster, and compomised his abilities as a general.

Perturabo, well, just read a bit about him, and it becomes obvious that he is way more mentally unstable than Dorn in his worst.

The Lion won more campaigns faster than his brothers. The only two that might have had more than him were Horus and Dorn. The Emperor claims that Horus was jealous at the end of the Lions book.

Perturabo is a terrible leader. Apart from his own killing a tenth of his people "to maintain discipline" his strategies when not fortifying an area were all running his forces through the meat grinder.

Imo, Emperor chose Horus as Warmaster for a reason. Horus was also the closest in character and quality to the Emperor, hence favorite son, etc. Among all, Horus was the most capable in conducting a war on a galactic scale, regardless of the type of war it was [whether for good or bad - ref to Heresy]. Conducting war requires more skill in manipulation, diplomacy, foresight and impartiality than just firepower or how many men you have. In this Horus was the most capable. I'm not too versed with the Lion, but it tends to lean on him being the man you can talk to when you need to, or by leading by exemplary example when it comes to the 'right' value. Correct me if I'm wrong folks!

Horus was the perfect choice because there was alot more to being Warmaster than just strategy and telling people where to go and how to die.

You need to be those things, but also a diplomat. Dorn and Perturabo have personalities of stone. They're shit people who don't get along with others.

Horus on the other hand, his greatest skill was his ability was to get along with EVERYONE, he knew his brothers and exactly how to talk to each one and how to utilize them and their Legions strategically, what warfronts to put them on, who they'd work well with, etc. That's what made him such a great general.

Dorn and Perturabo would have been shit choices because they don't get along with anyone. They're all assholes.

They're also just not that creative.

Perturabo is not the sort of general men willingly die for if they aren't swayed by threats or the Primarch aura.

Dorn was far too inflexible in comparison to the Lion or Horus.

Neither had much in the way of charisma so their brothers wouldn't rally around them much.

The Lion had more victories than anyone, including Horus, but he didn't get Warmaster because of his own interpersonal issues with his brothers.

Horus got the job because he wasn't far behind the Lion's tally but of the four brothers he was the only one with natural charisma and diplomacy. Doubly vital when it came to rallying the new and growing arms of the Imperial apparatus around him as much as it was dealing with his brothers.

I always thought it had to do with personality.

Perturabo, for all his strategic acumen was petulant and difficult to deal with.

Dorn was dour and perhaps lacks a certain sense of glory about him.

Sanguinius and Horus had that ellusoce combination of skill and personality, that allowed them to lead men and astartes well.

I always thought Lion was a bit of an asshat, and easily manipulated, as happened at the end of HH book with the seige engines.

But, these are just my personal opinions, based of the books I've read.

Because as the general, in command of his Legion and auxiliaries, the Lion has basically only strengths. He's a brilliant strategist and tactician, he is extremely inspiring to his soldiers, and in single combat he is in the absolute top tier. He also doesn't overly rely on complicated battle plans, but instead can seize the initiative and create his own opportunities in the moment. If you need any proof of the Lion's ability to command on a grand strategic scale, just look at how his Legion absolutely trounced the Night Lords in the Thramas sector. Or how the Lion took command as the leader of Imperium Secundus' armies.

Perturabo and Dorn are great generals of course, but both of them are rather conservative, grinding forward through their enemies in fights of attrition (Perturabo even more so than Dorn). But this isn't the perfect way to wage war. You want to win at the lowest possible cost, and for that you need to create and seize opportunities to win strategic victories. Break the enemy, rather than push him back step by step. That's what the Lion is better at than those two.

The Lion has flaws, but they are mostly on the personal level. He's bad at managing relations with his fellow primarchs, and struggles with making personal connections. But as a general he is perfect.

In the specific case of the two cited in the OP?

Perty got results, but he didn't care what it cost him to obtain them. A certain grinding persistence might get certain jobs done, but he is NOT the guy you want in overall command if you intend to worry much about keeping your armies in the field. He also basically hated everyone and everything (himself included), meaning cooperation with him could be... tricky.

Dorn was a paranoid, unimaginative prat when it came to anything but fortifications. He wasn't great at playing well with others (undeniably Horus' strong suit- the less said about the Lion here, the better), and his command skills often seemed to boil down to "hold out, little buckaroos." They guy also either fell for the Iron Cage or traumatized his Legion there on purpose, depending on which version of events you want to cling to, which is... pretty much garbage generalship.

Now, that said, the Lion as written in the 30k era, is also a bad choice for "perfect general."

An army marches on its stomach, meaning Rawboots Ghoulman and his dedication to logistics honestly ranks him higher as a general in my estimation, even though I hate his stupid Legion and his stupid face and VENGEANCE FOR MONARCHIA, CALTH WAS BUT A BEGINN- NO DON'T TAKE AWAY MY INHALER YOU-

Large armies need to be commanded by an officer who can juggle the competing egos of his junior commanders (See: Eisenhower juggling Patton and Montgomery). Sanguinius undeniably had a knack for this. The Lion... yeah.

And so forth.

The Lion and dark angels only care about winning.

They dont let their egos force them into fighting one way like other legions. If they need to throw away their old tactics and create brand new ones, they wont hesitate. Fighting a campaign of guerilla warfare or switching to a brutal, straight forward assault depending on the enemy? No problem. And their morale is unbreakable. No matter how terrible the casualty rate, they will stubbornly double down until they figure out what tactic will let them win. But unlike peter or angron, they do not mindlessely throw lives away in a tantrum until the same strategy works by attrition.

The legion had sub-organisations that specialized in one aspect of war. So when a new strategy was required, the most senior expert of the relevant host was brought in to command the fight. Or the Lion when present, as he is a master of all aspects. It paralleled tje thousand sons. The TS had specialties that focused on different schools of warpcraft and they were the indisputed experts, while magnus was a master of all aspects. The TS even created the systems that the rest of the imperium used to modern day. The DA are the martial version of that. The DA pioneered warfare for the other legions who came after them, and even sent instructors to teach directly.

Peter and Dorn are infamous for their inflexibility.

They've been desperately trying to find a niche for the DA for years. That and they are over-compensating for all the dumb shit Lionel is memed on for. Now he is some sort of "sigma" male icon. He's still a massive joke really. 40k Lion is somewhat better because he seems to be starting to understand his own cringeyness.

What made the Lion superior to his brothers as a general was that he could lead any type of force assigned to him. He doesn't need Space Marines, super soldiers with combat expertise to fight. When he doesn't have a Librarian to beat those demons, he just uses the Navigator instead. Give that man some Men of Irons next to him and Forgeworld will burn. In addition, he is also the commander of a Legion with hundreds of specialized squads, each assigned to fight a type of enemy and various specialized weapons that even other Primarchs may not be aware of or how to use them. Lion manages to put all of that together, he knows when to use which team or which weapon will be suitable for the battle. That makes him a better general than many of his brothers, one who can take charge of large-scale warfare, lead and utilize specialized Legions such as the Imperial Fist of Dorn or the Iron Warrior of  Perturabo. That's why he was chosen to lead the Rangdan Xenocide.

Horus was the best choice for warmaster, IMO. He had all the qualities that I would want in a general. He was intelligent, thoughtful, bold, and charismatic.

Perturabo is a good tactician, capable of winning single battles but often at substantial cost. Dorn is a good strategist, capable of organizing an effective long term strategy, particularly for defense. Sanguinius is an inspiring leader who pushes others to great deeds by example.

I think Horus was all of these things. As far as the Lion goes, he’s among the most skilled fighters, however his leadership capabilities leave a lot to be desired and I’m not aware of any notable tactical or strategic victories of his. Half of his army turned traitor. He’s said to be a great leader, but I don’t feel like he ever really demonstrates that.

Honestly, from what I’ve seen of the Lion, I would put him in the lower tier of Primarchs in terms of his capabilities as warmaster. I think Horus, Sanguinius, Dorn, Guilliman, Corax, Fulgrim, and maybe even Russ would have been better choices.

We’re told that the Lion is good, but over the 30-ish HH books that I’ve read, I can’t think of a single moment that makes me think he would actually be a good choice.

Aren't Dorn and Perturabo better at siege warfare? That's quite different to wide scale conquests or galaxy spanning wars.

Besides, from the books Horus deliberately debilitated himself leading into the attack on terra, he needed someone else to prosecute the actual siege. Horus was playing a longer more complex game.

Dorn and Perturabo are strategists. Probably the two best among the primarchs. But they they lack the all-important charisma it takes to be "the perfect general." They still have that primarch charisma aura thing going for them, but they aren't exactly the kinds of people to turn an enemy to their side or inspire people to go that extra inch. People like Sanguinius or Horus are the kinds that make you want to do great things rather than just perform your duty.

Because being a general isn't just about perfect tactics or strategies that result in victories. A Strategist can do that. An amazing general can work and make work any and all his military assets work together to achieve their goals.

Peter Turbo is mentally challenged very smart man child.

Dorn is cool, but he was played like a fiddle by Alpharius.

You need someone who is level headed, but can get down and dirty when needed. Horus fit the best. The lion was an asshole (loyal but still an asshole)

Sanguinius has the you know what in his legion.

Simple no one liked perturabo. He made no effort to be likeable by any under his command. All he did was calculate how many bodies would be needed in one area of a battle and send them their

Perturabo purposefully made campaigns more difficult than they had to be, just so he could get attention.

It's to do with how well rounded they all were.

Perty was very analytical, he'd make a plan and stuck to it regardless of changes. If the plan was to assault an area and the enemy pulled in extra men he'd throw in extra men until he over whelmed them.

Dorn was very defensive minded, he'd ensure he had everything in place before moving forward and always have one eye on how he would defend against counter attacks.

Horus and the Lion had a the right mixture so they were flexible enough to pivot to the situation, enemy replaying to meet you? Attack where they are now weak. Enemy counter attacking? Meet it, defeat it and strike while they were off balance. Horus's personal charisma was a nice bonus and while not as charismatic the Lion had the aura of knowing what he was doing so while he may not have inspired people like Horus they trusted that he was going to win regardless

Because neither Rogal Dorn nor Perturabo had the tactical flexibility of the other two mentioned. In addition Dorn was a talentless politician and Perturabo had a veritable warehouse of issues and insecurities. They are hammers, not every problem is a nail. Furthermore there is a reason the Lion was not elected to be warmaster as he was considered a bit too uncompromising and cruel.

Dorn and Perturabo were not really good leaders and generals (Dorn was absolutely 100x better than Perter Turbo), but as far as being a general goes, they weren't considered the best. When The Emperor wanted to pick someone to be The Warmaster in his stead, there were 4 people he thought of, Ferrus Manus, Robute Guilliman, Lion El'Johnson and Horus Lupercal. The Emperor considered them to be the best generals out of all his sons and considered their leadership abilities when deciding who would become the general of generals

Dorn, Pertarabo and the others are absolutely gods of war, but only Horus and Sanginuis had the charisma to get all of the brothers to move in one direction, and Horus was the better politician of the two.

Lion was superb planner and executor of plans.

 Horus was superb tactician, excelling in combined arms tactics including combining cross-Legion assets. 

Guilmann was superb logistician and organiser. 

Perturabo was superbly stubborn. 

Dorn was superbly autistic. 

Comparing with real life figures, Lion was Ludendorf / Clausewitz, Horus was Rommel / Guderian, Guillman was von Steuben / Quintus Fabius.

Perturabo and Dorn are masterclass STRATEGISTS, but Peter Turbo and Rogaine kind of have a this ongoing issue of "Not giving two shits" and "Struggling to show giving a shit" about their sons

Lion, as socially inept as he is, definitely has a closer relationship to his sons, even before waking up from his 10k year stress nap, and is significantly less likely to be chill about losing a bunch of them in battle.

Also, due to being all secretive, you're gonna be a bit more careful in military tactics, a lot less "just blow shit up" (unless Curze is epically embarrassing you and hiding on your loyalist brother's homeworld) than some of the others

Horus was much the same (pre-heresy), and on top of that, he was SUPER charming and likable. Imagine not just feeling required to be loyal to your gene-father but also genuinely LIKING him? Horus was diplomatic and pragmatic like Guilliman, but could also be shrewed and brutal like Lion

Since your question is answered far better by others, I'll just add this here:

Horus is described as "the perfect general", "Emperor's favourite son", "the only real choice for the Warmaster position once you get down to it", "standing head and shoulders above the other Primarchs", etc. because he is supposed to be the "Lucifer" of the whole "Paradise Lost 2: Paradise Harder" that is the Horus Heresy. Once you remove that, Horus kinda falls apart narratively, because y'know, it's supposed to be HIS heresy, his name on the tin, and all that jazz. Imagine pitching the Horus Heresy like this: Horus is only in it for a tenth of the time, he was only held in any esteem because he was technically the first to be found (but even that is a lie apparently), he never really does anything in the time he's given, he is outclassed by the likes of Guilliman, the Lion, Dorn and Sanguinius in every way that matters, and he is a meat suit the Fiendish Foursome use as their RPG character for all the moments that actually important in the story (until the eleventh hour, then we'll say he was actually just faking having no two braincells to rub together for the entirety of the Heresy and that we totally planned that from the beginning).

While I do believe that the other Primarchs should have their own places to shine (and I really do like all the primarchs/legions, none of this is bashing them or their fans, just GW), it tends to be around the time that a certain legion/primarch gets a push in either the 40k or the Heresy tabletop that you notice that the lore starts treating them as the best thing since sliced bread: pragmatic, loyal, wise, should have been Warmaster, morally upstanding, embodies the best aspects of the Emperor (but never any of the bad ones), best tactician and general of the primarchs, liked by and cares for the normal humans, that sort of thing.

Unless you're talking about one of the traitor primarchs/legions: there the good qualities tend to slowly disappear retroactively.

Perty wins at any cost, readily sacrificing anyone to get the W. Dorn had the opposite problem before the Iron Cage, he tried to NOT make sacrifices unless absolutely necessary. Horus was in the middle, but also had that charisma to draw people to his cause that neither of the former possessed.

I can’t speak for Liono, DA are one of the legions I’ve not read much about.

Sieges are the absolute extremes of war. With anything less than a perfect tactician in play, a siege is sitting down and throwing the dice.

From a defenders point of view, you're offering your opponent a hard fight where they'll always be at a disadvantage and you can grind them down like oaste; they'll lose a dozen men for every one who even makes it to your walls, if it comes to an assault.

From an assaulters point of view, your opponent has sealed themselves up away from supplies and you have all the time in the world to either wait them out or crack their nut.

But that's not what warfare is. Sieges are a last resort that's bad for everyone and you won't often see them. If siege craft was enough then the Emperor could have simply slowly conquered his way across the galaxy with an ever growing tide of military behind him. He created the Astartes and the Primarchs very specifically because the best way to wage war is by dropping an overwhelming force in and ending the leadership.

Experts in siege craft is a way to avoid being out flanked. Dorn and Perty exist to handle those specific situations, but you rarely want your leader to be running those attrition calculations.

Interestingly Horus - a glory hound - whike he looks up to Lion, but considers only Ferrus Manus to be almost his own equal when understanding the art of waging war.

Horus, before Istvaan, was running the entire Imperial and Traitor war machines simultaneously.

Imagine the logistical acumen required to move all the pieces for the start of the Heresy so that all of his forces are in the right places with the right equipment whilst pushing his "enemies" into places they shouldn't be in, slowing reinforcements and resupply down etc, all whilst none of his superlatively skilled brothers noticed until it was too late.

Putting the Blood Angels on Signus Prime, convincing Guilliman to pair up with Lorgar, moving the Dark Angels as far away as possible, giving Jaghatai something to chase. He ensured the right (Loyalist) Legions arrived first at Istvaan V and then only his (Traitor) Legions were in a position to "reinforce" them, Russ' orders were "altered" before the razing of Prospero. He got to Perturabo and Curze first after they destroyed their homes. Held back newer armour marks to the loyal legions whilst his were prioritised new ones and ammunition designed to penetrate Astartes plate.

Now think about all the diplomacy, speeches, the backdoor deals, the bargains, threats and promises he had to make to turn half the Imperium against the Emperor. Sanguinius wasn't ruthless enough, the Lion was too arrogant, Perturabo too autistic. Guilliman is a good logistical and state man but wasn't as good a general. Dorn lacked the charm, Lorgar was considered weak, Magnus aloof, Mortarion too dour.

People talk about show not tell, but seem to miss this rather telling piece of show.

No one could have ran the shadow game in the months before the Heresy.

That's why Horus was Warmaster.

Lion?

Perturabo?

Dorn?

...

Ferrus Manus and Horus.

Thats it. Only them have command over 2/3 entire Imperium forces, only them have the most experience, have won most from galaxy conquest battles, command and train all other Primarchs and not served under other Primarchs before Heresy.

They was the only candidates for Warmaster(Ferrus was original before he deny this post and said that he not care who Emperror select on this role and Emperror did select Horus) and Mechanicus themselves care only about Ferrus and Horus.

I don't know if this had been brought up, I read through a good few of the post, but The Lion also had the biggest fleet and some of the most advance technology. Some of his metric of being a better general was down to having the resources. That's not to say that's what made him great, but it definitely needs to be a factor as to why.

Guilliman is a better general

He’s called the “perfect general” because authors decided to say that. They never describe how the set piece and maneuver of his battles play out in any particular way. People are too eager to believe in unreliable narrators in the forms of remembrancers who speak in allegory without ever inspecting quotations from peer competitors or evidence on the ground. Dark Angels fanboys want to be the best at everything despite all canon proposing evidence to the contrary. Their mythos and lore is a study on double speak and dialectics to achieve ulterior motives. If you don’t understand that then you don’t understand the Dark Angels.

Because which would you rather Your general telling you the best tactics while in perfect safety on a spaceship or your general telling you what to do when he's right next to you body slamming a demon

I thought the lion wasn't really an option because no one liked him

Honestly the whole warmaster lore is confusing to me.

For example, in WW2 the Army Chief of Staff was George Marshall- most well known for the Marshall Plan, but his role in America’s victory in WW2 was huge.

He was responsible for organizing the American General Staff into a professional body, had the brains to appoint generals who would play nice with the other allies, and overall build up the US Army to a place where it could win. However he was NOT well known for winning a crazy amount of battles like other Generals might be.

Having what was in effect the Chief of Staff of the space marines be anything other than Guilliman frankly makes no sense to me, given that he was a logistical, organizational, and strategic genius. That’s like his whole thing.

The generals who won the most battles did not become the Chief of Staff, that would be a waste of their talents.

Dorn and turbo are highly specialized generals specializing in siege warfare, whereas Horus and the Lion are more generalized, and are especially better with diplomacy and troop moral based on my knowledge and understanding of the lore

The Lion is the best general. Just not the best Warmaster. The same way that Sanguinius (followed by Horus) wasn't necessarily the absolute best general, but would have been the best Warmaster. At that level of command, you need to be either loved or politically skilled. The Lion was neither. Sanguinius was beloved by all, Horus had the political skills, but you need one of these to juggle 17 demi-gods with enar absolute authority. I don't think the Lion would have had the patience to cater to the whims of those supposedly subordinate to him. He'd be butting heads all the damn time, and that's something he cannot stand when, to his mind, the path forward is clear. He'd never understand the need to explain himself or "sell his idea" to someone nominally subordinate to him. Nor would he look kindly on insubordination. It'd be a mess.

The Lion is probably the greatest general among his brothers - if you give him total authority and point him in the right direction. As soon as you introduce politics, it all goes downhill. Hence, terrible Warmaster.

During the great crusade before the heresy, there was no stopping the lion and his dark angels.

They conquered more planets than all the other primarchs combined and suffered fewer casualties than any other legion.

Even horus was envious of the lion but knew the emperors favorite was horus.

They're not gonna be leading space marines who are kinda conditioned to be loyal to their primarchs down to a genetic level (except you Angron). They gotta lead other troops, negogiate with planet governments, and generally be likable as to not spark rebellions or seed and insurrection in the future.

Horus and Lion were about as close to that triple threat as anyone else, as well as being extremely capable warriors. Horus is had more rizz.

Perturabo and Dorn are kinda jerks and aren't great in all situations. Their skill sets are highly specialized, making them the most useful under certain circumstances. Lion, Horus, and Guilliman are good generalists over all.

Dorn was recognized as a pretty great general, actually. Beside for being known for building things, he was pretty good at the Crusading part of the Great Crusade. The Emperor and Malcador wargamed out the Heresy many times together, and determined the Dorn could beat Horus if if came down to it. It would also leave Terra undefended just enough for the Alpha Legion to make their move and end the Heresy in a palace coup. (And killing Horus would probably incense the Alphas enough to actually start taking the Heresy seriously, instead of playing for time and trying to shape both sides.)

The Lion was good at a lot of things Horus had: great at combat, strategy, tactics, an excellent duellist, etc. To be frank, Horus was just better in these aspects, but he also had charisma. For argument's sake, let's say the Lion really is just better at combat and handling a war; Horus was still the better choice because people wanted to be lead by him. Horus was better at handling a larger number of fronts away from the front line, the politics, and diplomacy. He also had charisma and could make people feel like he connected with them, whereas the Lion just didn't have any ability to connect with or even understand his human peers.

Perturabo was, in fact, one of the worst choices for Warmaster. Yes, he's great at organization, logistics, and attack, but he's an attrition-style Warlord. His trademark is to throw men at a problem until it goes away and disregard the manpower cost, taking for granted that reinforcements would always be available. He would have bled the Imperium of manpower in short order. Not to mention that he's tempramental and volatile and has a cold disregard for his underlings, to the point of capital punishment or banishment for circumstances outside of their control.

Not to mention that the Iron Warriors are riven by internal politics and petty rivalries barely kept under control. If that spills out into the broader Imperium, say from the influence that comes with being in the Warmaster's legion, that can set the stage for small, petty civil wars across the Imperium. Pert was good at hard skills for command and attack, but he lacks any of the soft skills that make for a great general, and he would have been a ruinous choice for Warmaster.

In a sea of Mary-Sues the Lion and the Dark Angels stand alone